The human knee is an example of a joint that is capable of complex motion including flexion/extension, adduction/abduction, anterior femoral-tibial shift, and axial rotation. Due to the inability of damaged cartilage to repair itself after injury, the typical treatment for patients with degeneration of the knee is the undertaking of a total knee replacement with a patello-femoral-tibial prosthesis.
Successful knee replacement with a prosthesis requires that the bearing surfaces allow, as much as possible, the knee to have a natural range of movement as determined by the ligaments and soft tissue surrounding the knee. This movement includes flexion/extension, roll back of femur on the tibia and finally rotation of the femur on the tibia at all angles of flexion.
One solution to date has been the “meniscal knee” developed by Goodfellow and O'Connor in Oxford, England (see U.S. Pat. No. 4,085,466). This knee provides a three part articulation comprising a curved femoral surface, a flat tibial surface and a meniscal bearing between the femoral and tibial surfaces. The meniscal bearing is normally formed of a suitable synthetic plastics material such as ultra high molecular weight polyethylene. It has a flat lower surface that slides on the tibial surface and a concave upper surface that receives the femoral surface. Constraint, and the shear stresses that go with it, is generally avoided as the meniscal bearing slides about on the flat tibial surface as the knee flexes, rolls and rotates.
While development of the meniscal knee has been well received there are a number of difficulties still inherent in the placement of prostheses of this and other types. The surgical placement of such prostheses has to be very precise and technical failure for this reason is common.
Accurate bone cuts are important for successful knee or other joint replacement surgery. The accuracy is assessed in terms of the flatness of the cut (which is a function of the stability of the saw guide and the accuracy of the saw system), the orientation of that cut within the body, and the accuracy of the cut with reference to other cuts. These latter two are most susceptible to improvement through use of guided and navigated orthopaedic surgery.
Navigation and robot systems are becoming an increasingly appropriate technology for use in knee and other joint replacement. For example, several systems exist for navigation of the knee to aid alignment of the standard total knee bone cuts. In use of such systems, in some cases the first step is for the patient to be scanned pre-operatively using computer aided tomography (or CT) techniques. In this way, the three dimensional “form” of the patient's bones is stored in a computer's memory. In addition, three dimensional data for the knee implant to be used by the surgeon, or for a range of possible knee implants which could be used by the surgeon, may also be stored in the computer's memory.
During the knee replacement operation an image of the bones is displayed on a computer screen so that the surgeon and others involved in the joint replacement are provided with the necessary information to perform the surgery. Alternatively a process to register the patient's actual bones from particular landmarks may be undertaken. This is typically performed after sensors or markers are fixed to the bones to coordinate the screen image and data capture with the position of the patient's bones. Bone cuts can then be made to suit the implant after sizing and judging the positions of the components on the computer screen. In addition, kinematic information may be gathered once the bones are “registered” allowing range of motion and leg alignment to be documented.
This technique can assist in ensuring that each of the components of the implant has controlled orientation relative to the bone onto which it will be implanted. This “landmark” technique replicates most existing systems for knee replacement that do not involve such a computer.
The use of sensors or markers attached to the bones in navigated surgery can be a problem due to the fact that the required holes to affix the pins securely holding the sensors are a potential stress riser and a track for infection.
Cutting guides are also typically employed to ensure that the bone saw used to resect bony tissue performs resections corresponding to mating surfaces of the prosthetic component. For example, in a femoral knee replacement, cutting guides or blocks are temporarily secured to the distal end of the femoral shaft, and include slots into which the blade of an oscillating saw is inserted to shape the end of the bone in accordance with corresponding surfaces of the prosthetic element. These cutting guides are also affixed to the bone temporarily, yet securely, with pins or screws.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.